Modern glass-making process requirements have placed a greater demand on the performance of materials used for glass-making molds. For instance, glass quality requirements are greater, process temperatures are higher, closer control of dimensional tolerances is desired, longer service life is expected, and high productivity has become an economic necessity. All of these requirements have pushed the demands on the properties and performance of mold materials to higher and higher levels. This is more so prevalent in the precision glass making industry as the growth of the lens market in consumer electronics, for example camera phones and digital cameras, and industrial optics has shifted lens production from traditional diamond turning operations to high volume, low cost molding operations.
In addition to improving the quality of the mold material which in turn improves the quality of the molded glass, increasing mold life is also desired. Examples of factors that affect both quality and mold life is the chemical inertness as well as the machinability of the mold material. In particular, precision glass lens producers report chemical interaction of the hot mold material with the molten glass during molding operations as being one of the primary causes of mold failure. This problem of a contaminant in the mold not only decreases mold life but also diminishes the optical purity of the glass or lens being produced. An example of such a contaminant is nickel which can diffuse into the glass/lens during production. One solution that has been utilized to prevent such diffusion is the use of a coating layer on the mold to minimize the interaction between the mold and glass. However, even with the presence of a coating, typically less than 0.5 microns in thickness, it is still possible for a contaminant (iron/nickel) to diffuse through and react with the glass. Thus, while helping to delay the mold-to-glass interaction, the presence of a coating has not, to date, been successful in preventing the diffusion of contaminants.
In addition to the problem of contaminants diffusing through to the lens or to the mold during the molding operation, dimensional accuracy and surface finish of the mold material can also have an effect on mold life. Material imperfections which are revealed during final polishing add additional costs to the manufacturing process in the form of reduced tooling yield and expenses related to rework. The machining of aspheric shapes in molds renders the molds relatively expensive, particularly since very hard and durable mold materials are generally required.
An array of solutions has already been considered in the industry for addressing the problems of mold life, fabrication costs as well as quality of the molded glass. One solution includes the use of a high chemical purity silicon carbide material. While the chemical inertness and high hardness of silicon carbide make it a material of interest for precision glass molds, the brittle nature of silicon carbide can present handling and finishing concerns. Furthermore, silicon carbide is often an expensive material solution and therefore is not practical.
Another alternative may be the use of ceramics. The relative inertness and high hardness of ceramic materials, such as silicon nitrides, are beneficial for applications such as glass molding. However, final grinding and polishing can be time consuming and expensive due to the parameters required to obtain the required surface finish without chipping and/or breaking. More importantly, the co-efficient of thermal expansion in ceramic materials is significantly lower than that of the glass being molded and introduces mold design challenges.
As mentioned above, coatings may be another alternative. The application of coatings which are resistant to oxidation and wear have been proposed for glass molding dies. Coating adhesion is a consideration in all coated tooling applications. In applications in which tooling can be refinished, or dressed, the thickness of the coating must also be considered.
Another option is a binderless carbide. This solution has been discussed in the industry as a good fit for precision glass molding applications due to the high hardness and matching coefficient of thermal expansion of tungsten carbide. It is understood, however, that achieving full densification in the absence of a binder material presents a significant manufacturing challenge, leading this type of material to have microstructural defects which in turn renders it unsuitable for finishing as well as subsequent usage for mold tooling.
It also must be understood that diffusion of the metallic binder which, in theory can be eliminated by making the material “binderless”, is only one proposed reason for degradation of the glass during the molding process. The inertness of the mold material to the glass can also be dependent on other factors such as overall chemical composition, impurity level, and chemistry and microstructure of secondary phases that may be present in the material as a result of either the material design or processing.
It has also been reported that anomalous phases containing metallic and carbide constituents (known as inclusions and anomalous phases) are sometimes present in the material. These phases are often associated with clusters of porosity, and their presence impairs grindability and performance of the material. Origin of these constituents is generally hard to trace, and improvements in the material design would be needed to eliminate these unwanted phases.
To successfully address the functional requirements of precision glass molds, technical solutions need to consider both the advantages and challenges of specific design concepts which balance inertness with machinability. Accordingly, there is a need for a cost effective material system for precision glass molding operations that is chemically inert and can be finished to a nanometer surface finish.